The invention relates to the field of non-destructive arthroscopic diagnostic probes, and in particular to non-destructive arthroscopic diagnostic probes for detecting degeneration of articular cartilage utilizing impedance measurements.
The function of organs in the human body are a direct consequence of their inherent structure. The function of an organ as a whole is more than the sum total of its individual constituents. Articular cartilage (AC) is a rich and illustrative example. An understanding of the composition and physical properties of AC are essential to diagnose a disease with any given device to aid in patient care. AC is a dynamic, living tissue that responds to stimuli in its environment (i.e. external loading, fluid flow, electric fields), and the cells of cartilage (chondrocytes) are able to maintain its intricate extracellular matrix (ECM). The scientific data collected over the past 25 years for normal cartilage, supports a hypothesis that a feedback between mechanical stimulation and chondrocytes must exist to maintain cartilage homeostasis.
By gross visualization, during knee arthroscopy or an open joint procedure, normal AC appears as a homogeneous shiny white substance covering the ends of articulating bones. It is a thin layer from 1 mm to 6 mm depending on the joint and particular surface location. In the presence of synovial fluid, AC provides a very low friction surface that has a coefficient of friction that is less than that of ice on ice. A closer inspection at the light microscopic level reveals AC as a very complex ECM of macromolecules with chondrocytes embedded within. Cartilage is a unique organ as it is aneural, alymphatic and avascular. Nutrient exchange to the chondrocytes proceeds by diffusion from synovial fluid at the articular surface and from the subchondral bone below. The lack of a blood supply severely limits AC""s ability to be repaired following injury. The absence of innervation means that pain is transduced from the surrounding bone through unshielded force, or from the joint capsule in response to an inflammatory stimuli response.
The complex structure of AC acts as a loadbearing, shock absorbing, and wear resistant material to protect joint surfaces. In addition to a low friction surface, AC has a high compressive strength critical to pain free joint function. Compressive loads are distributed over a larger area, while also acting as a damping element during high impact loading (i.e. jumping). Cartilage is also strong in tensile loading when subject to shear stresses due to the sliding nature of joint function (i.e. knee joint or intervertebral disk). Lubrication by the synovial fluid also reduces shear stresses and helps protect the cartilage from trauma. These macroscopic mechanical properties are a direct consequence of the composition.
Articular cartilage is mostly water (60-80% of total weight) and ECM that comprises the bulk of the dry weight. The primary structural components of articular cartilage (AC) ECM are produced and maintained by the chondrocytes enmeshed within it. Tissue mechanical properties depend on the organization and structure of macromolecules present in the ECM. The ECM is made up of mainly collagen type II fibrils (along with small amounts of types IX and XI collagen), charged proteoglycans (PGs), and cells. Collagen and PGs form the framework for cartilage that resists applied mechanical forces. The collagen forms a dense cross linked network with PGs embedded within. Proteoglycans are macromolecules that contain polyanionic sulfated glycosaminoglycan (sGAG) chains. The negative fixed charge density of sGAGs is approximately 5.3 mEq/gm dry weight in normal human femoral head cartilage. A slight excess of mobile positive ions within the tissue preserves electroneutrality. At the macroscopic level a Donnan osmotic swelling force develops, caused by the electrostatic charge repulsion between the fixed anionic groups that draws water into the ECM, expanding the collagen network.
The chondrocytes are responsible for PG turnover (synthesis and degradation). The most abundant PG is aggrecan, which has an extended protein core with up to 150 chondroitin sulfate and keratan sulfate chains attached in a xe2x80x9cbottle brushxe2x80x9d structure providing a high concentration of anions. When first synthesized, aggrecan is mobile, but quickly binds to immobile hyaluranon, stabilized by a link protein, creating the high density of fixed COO31 and SO3xe2x88x92 groups at physiologic pH.
Many other soluble factors play an important role in the maintenance process by participating as mediators of turnover and production of ECM, including ions, growth factors, hormones, cytokines, proteinases (e.g. matrix metalloproteinases) and their inhibitors. Numerous factors are required to maintain homeostasis. They can be produced by the chondrocytes themselves or synthesized elsewhere and transported into the ECM. These factors affect the chondrocytes through cell surface receptors and their transport through the ECM can be prohibited, resulting in pathology.
At the ends of articulating joints, the AC is 3-4 mm thick, with areas on the patella as high as 6-8 mm. Microscopically mature AC has 3 zones based on the shape of the chondrocytes and distribution of the type II collagen. The tangential layer has flat chondrocytes, tangential collagen fibril orientation and a sparse PG content. The intermediate layer is the thickest, with round chondrocytes, oriented in vertical columns. Finally, the basal layer has round chondrocytes and contains the tidemark that separates the uncalcified (nourished by the synovial fluid) and calcified cartilage (that gets fed by the episphyseal vessels). It has been reported that no age changes after maturation are discernible based on histology, including no loss of AC.
Collagen makes up the majority of the dry weight (approximately 50%) of AC, as it is also the most common structural protein in the body. In cartilage the most abundant form of collagen ( greater than 90%) is type II that acts as the structural meshwork of the ECM with its associated extensive intermolecular crosslinking via trivalent hydroxylysyl pyridinoline residues. The name xe2x80x9ccollagenxe2x80x9d is a generic term for structural molecules that are rich in glycine, proline and hydroxyproline. Striated fibrils, type I, II, and III have three polypeptide chains wound in a triple helical configuration.
Type II collagen is composed of three left handed tightly interwoven alpha chains (xcex1(II)1), 300 nm long and 1.5 nm in diameter., each with a repeating amino acid sequence of GlyPro(Hydroxyproline). It is the triple helical structure that enables collagen type II to have a high tensile strength. Hydroxylysine (some as hydroxylysyl pyridinoline crosslinks) helps type II collagen link together the ECM network.
Small amounts of collagen type IX help connect the various matrix elements together while type XI (approximately 3%) regulates the caliber of the fiber. In addition, collagen types VI and X are also present ( less than 1%). Collagen type VI has a cross linking behavior and an increased amount has been reported in OA models, while collagen type X is associated with growth plate cartilage in the hypertrophic zone, and in the calcified layer of mature cartilage.
Individual aggrecan monomers are attached to a GAG core (hyaluronate), stabilized by link protein, with the number depending on the functional nature of the cartilage. The common PGs that can be found in cartilage include aggrecan, decorin, fibromodulin, and biglycan and make up about 35% of dry weight of AC. Aggrecan molecules form large aggregates (approximately 200 MDa) in cartilage, forming a hydrogel-like structure that is, in turn, immersed within the collagen type II fibers.
Aggrecan is the major PG in AC (around 90%) and is composed of two types of sulfated GAG (sGAG): chondroitin-6-sulfate, chondroitin-4-sulfate (approximately 20,000 MW) and keratan sulfate (approximately 5,000 MW). The amount of sGAG attached to each PG varies depending on the functionality and integrity of the tissue. Chondroitin sulfate GAGs are chains of repeating disaccharide units that contain highly charged carboxylate and sulfate groups. The high density of negatively fixed charge groups helps attract positive ions and create an osmotic swelling pressure to imbibe water within the tissue. There are approximately 100-150 sGAG chains per aggrecan molecule, while extremely large aggregates can bind thousands of GAG chains.
At the molecular level, the main unit of the aggrecan molecule is a protein core of approximately 300,000 MW. It has 3 associated globular domains: G1 and G2 at the N-terminus, and G3 at the C-terminus. The GAGs are mostly contained within the G2 to G3 domain. Keratan sulfate chains bind, in general, closer to the G1-G2 region (interglobular domain). The aggrecan PG varies in total size due to the varying amount of bound chondroitin sulfate. Therefore, proteoglycan is a combination of 5% protein and 95% carbohydrate.
Variation in the concentration of PGs has been observed with depth from the articular surface in immature and mature tissue, as well as location within a joint. Areas bearing higher stresses have also been shown to have a higher PG content. The charge groups on the GAG chains are ionized at physiologic pH providing a large osmotic swelling pressure that largely determines the equilibrium compressive modulus.
The main function of chondrocytes involves replenishment of macromolecular ECM constituents for its preservation in its harsh mechanical environment. With respect to the volume of the cartilage, the chondrocytes account for less than 5% and have a density of approximately 20xc3x97103 cells/mm3. In fully developed tissue the size, shape and density of the chondrocytes vary with depth proceeding down from the articular surface. In general, the size increases, the shape moves from flat and elongated to spherical, and the density of cells decreases with increasing depth toward the underlying subchondral bone.
In order to perform their biosynthetic functions, chondrocytes are well equipped with an extensive endoplasmic reticulum and Golgi apparatus as well as mitochondria and secretory vacuoles. Chondrocytes are also involved in regulation of ECM assembly and repair by secreting and mediating their assembly. Because of its ability to produce degradating enzymes as well as their inhibitors, the chondrocytes are believed to participate in the physiologic as well as in the pathologic degradation of ECM.
Chondrocytes have also been shown to be able to adapt to the changes by biosynthetically responding to chemical, physical, mechanical and electrical stimuli through their cell receptors. These adaptations, through balancing the homeostasis of the ECM, alter integrity to conform with the stimuli. Biomechanical stimuli of cartilage explants (static or small amplitude dynamic compression) have been found to influence the rate of aggrecan synthesis and catabolism. This behavior may be due to changes in cell shape, specific cell-matrix interactions or change the availability of growth factors.
Investigators are also now beginning to look past the cell membrane to examine the intracellular changes due to mechanical compression on MRNA levels and also how cell deformation affects intracellular organelles like Golgi apparatus or endoplasmic reticulum to fulfill their functions.
Water is the most abundant component in AC, and it appears to be compartmentalized. Water that exists in the interstices of collagen molecules and fibrils is intrafibrillar water with the balance in the extrafibrillar space. The distribution between these two compartments has been reported to be a function of the fixed charge density and loading configuration of the tissue.
The total amount of water present is dependent on the interaction between the collagen and sGAG components, as the collagen fibril reinforcements in the tissue prevent full expansion by the sGAGs (due to their fixed charge density) and thus constrain water intake. This balance is perturbed in OA cartilage. An increase in water content is observed compared to healthy cartilage, despite an observable reduction in GAG content. The explanation of this contradiction lies in the assumption that damage to the collagen network severely impairs its ability to restrain the sGAG swelling pressure (despite its lower concentration in the tissue), thus the amount of water increases. One of the hallmarks of pathologic cartilage is its increased water content.
Matrix metalloproteinases (MMP) are an important group of zinc containing enzymes responsible for the breakdown of ECM components such as collagen and PGs in normal embryogenesis and remodeling as well as in many disease processes like cancer, osteoporosis and arthritis. These enzymes are almost universally distributed among mesenchymal cells of all types, and in some epithelial and endothelial cells as well. The family of MMPs can be divided into 3 subclasses: collagenases (MMP-1, MMP-8, and MMP-13), gelatinases (MMP-2 and MMP-9), and stromelysins.
The collagenases MMP-1 and MMP-13 are members of the family of matrix metalloproteinases that play an important role in the degradation and turnover of the ECM molecules such as type II collagen and aggrecan during normal remodeling (e.g. embryogenesis) and disease processing. MMP-1 and MMP-13 have been implicated in the progression of osteoarthritis and rheumatoid arthritis since they are found in the associated tissues at higher levels than in normal human tissue. The active enzyme has also been found at the lesion sites on the tibial plateaus of Hartley guinea pigs during the progression of spontaneous osteoarthritis. MMP-1 cleaves the three chains of the type II collagen molecule at specific sites, producing helical trimeric fragments of xc2xe (from the N-terminus) and xc2xc in length that can be detected using neoepitope antibodies. MMP-13 initially cleaves type II collagen at this same site, but can additionally cause a second cleavage, residues carboxy terminal to the primary cleavage site, and then a third cleavage site another three residues carboxy terminal to the second cleavage site. The time course of damage induced by MMP-13 is more rapid and transient than that due to MMP-1; MMP-13 was also found to turn over type II collagen 10 times faster than MMP-1 in humans.
It has been recently demonstrated through immunostaining of neoepitopes in cartilage from patients with osteoarthritis that type II collagen degradation was initiated by MMP-1 and MMP-13 at the articular surface and then extended into the middle and deep zones. In addition to this depth dependence, a direct correlation between the Mankin score of OA cartilage degradation and the intensity of the immunostaining of MMPs was recently found.
As a consequence of the composition of cartilage, measurable electromechanical properties are exhibited (an area of research referred to as cartilage electromechanics). To consider modeling and analysis of AC behavior, AC can be conceptualized as a composite material of a fibrous mesh (collagen) embedded in a highly hydrated charged gel of PGs. Resistance to tension and shear loading is provided by the collagen, whereas the high swelling pressure of the PGs enables cartilage to resist compressive loading. The engineering properties of cartilage, i.e. compressive and tensile strength, have been assessed in many different systems. Extraction of PGs has produced a marked decrease in the tissue""s equilibrium compressive modulus without effects to the tensile stiffness, while selective degradation of the collagen network predominantly affects tensile properties of the tissue.
The dynamic behavior of cartilage is the result of interactions between the solid ECM components and the interstitial fluid. Cartilage is most successfully modeled as a poroelastic medium. Such a medium is a fluid saturated porous material in which viscous effects are predominantly due to frictional interactions between the fluid and solid phases. The literature documents a rich history of mathematical models to describe this physical behavior of cartilage. Early work was begun by Biot almost fifty years ago in the context of geophysics. Using a mixture theory, where material properties and constitutive relations are derived separately for the fluid and solid phases, a biphasic theory describing the behavior of cartilage was developed by Mow and coworkers. These models of fluid flow can be related to mechanical properties like stiffness. Interest today remains high, with continued efforts to produce more complex nonlinear models for use with today""s powerful computational capacity to more realistically model high strain behavior and the effects of nonhomogeneous material properties on experimental measurements.
Besides having mechanical properties, cartilage exhibits electrical properties that are coupled with mechanical stresses. It has been previously shown that these electrical interactions play a significant role in cartilage physiology.
The electromechanical transduction effect is a property of the cartilage composition, specifically the PGs and two dual phenomena are observed.
The first of these effects is known as streaming potential. As the hydrated ECM of cartilage is mechanically deformed, a flow of interstitial fluid relative to the fixed charge groups of the solid matrix is created. Entrained positive ions are separated from the negatively charged matrix macromolecules, giving rise to a voltage gradient or streaming potential in the direction of fluid flow. The second of these effects is the converse electrokinetic effect, known as current generated stress. This effect results when application of current causes an electrophoretic motion of the negatively charged fixed ECM molecules (PGs) towards the positive electrode and an electroosmotic motion of the mobile ions of the fluid phase towards the negative electrode. These combined effects produce a measurable bulk mechanical stress at the tissue surface that can be detected by an overlying stress sensor.
Osteoarthritis (OA) is the most common disease that directly affects the everyday mobility and quality of life. It mainly strikes in the last quarter of life, but also occurs in young people after traumatic sports injuries. It attacks the tissue of the synovial joints (e.g., knees, hips, and hands) and is characterized by pain and accompanied by limitations in joint motion. It can progress to endstage, when patients can no longer walk pain free. Although the disease itself is not a significant source of mortality, it is a great cause of physical suffering. In the US alone, estimates that the number of people suffering from OA will reach 68 million by the year 2010.
Osteoarthritis describes a group of joint disorders that lead to the destabilization of normal AC function. Degradation and synthesis by chondrocytes of ECM are uncoupled. The initiation of OA may be a result of a variety of factors, including genetic, metabolic, developmental, and traumatic. OA is diagnosed clinically with sharp stabbing joint pain, tenderness, and inflammation leading to limitations of joint movement.
One of the early events in OA at a molecular level is alteration of the cartilage ECM, and the loss of the highly charged macromolecules (PGs) from the matrix. These changes often occur in localized regions of cartilage along the joint surface and to nonuniform depth. Investigators have hypothesized that such molecular changes should change the tissue""s material properties.
Traumatic injuries can cause focal defects in cartilage adjacent to otherwise normal cartilage. Clinical repair approaches include debridement, microfracture, osteochondral plug resurfacing, and chondrocyte transplantation. During surgical procedures, and the subsequent follow-up, surgeons need to assess the state and functionality of the repair tissue. Often remodeling leads to a fibrocartilage repair tissue that appears cartilage like but has poor physical properties that ultimately lead to its failure.
In addition, there is a great need for methods to assess the efficacy of therapeutic interventions developed to prevent cartilage destruction or the patency of cartilage repair tissue. Presently, the assessment of cartilage repair is based on gross and microscopic morphological features. Detailed studies have established that the repair tissue is generally of good quality in the short term, but fails with time. At present, this behavior is difficult to explain. The literature shows only a minimal molecular characterization of the types of the cartilage repair tissue. The more that is understand of the repair process, the higher the chance to produce the optimal outcome; a repair tissue integrated in the native cartilage and biomechanically functional for many years. There is also a need for better in vivo, non-destructive diagnostic tools for quantitatively assessing degenerative changes in articular cartilage and to diagnose OA.
Current diagnostic criteria and methods for monitoring OA are based on external physical examination and x-rays. Efforts are being made to diagnose the disease progression at its earliest stages in order to apply treatment before further damage can occur. Initial diagnosis of OA begins with patients complaining of pain and stiffness in their joints. Further diagnosis can be made using the current gold standard of x-ray radiography. A grading scheme to characterize the damage is utilized; the most commonly used scale of radiographic evidence is the Kellgran and Lawrence method. The scale is numbered from 0 to 4 with 0 being no visible defects and 4 showing visible OA.
Radiographic diagnostic criteria has three main shortcomings: they lack sensitivity, the emphasis is on changes to the bone, and reading the films is subjective with poor reproducibility. Studies have shown that after 2 years of treatment with NSAIDS, the radiographs showed no significant changes. It is possible that the sensitivity is simply too low to detect small treatment effects. The limitations of radiographs include: nonstandard and shifting of joint positions, the x-ray beam alignment, radiographic magnification not taken into account and landmarks for measurement can be subject to individual interpretation. These defects have been corrected in current clinical trials. However, x-rays cannot visualize cartilage. They measure the space between bony surfaces that could be filled with cartilage and thus, are an indirect measure of the absence of cartilage or a failure of compressive resistance of existing cartilage. Thus, x-rays only examine cartilage status and can reveal bone and then joint involvement only in the later stages of disease.
Laboratory methods such as a synovial fluid extraction or a histological examination (Mankin scale) can be used to further investigate the progression of disease. Unfortunately, synovial fluid extraction can only rule out other possible causes for the pain, such as rheumatoid arthritis. The Mankin scale categorizes the extent of disease progression in tissue via histology but requires a destructive biopsy. Furthermore, histological examination occurs only where tissue was removed.
Other possible methods include MRI, sonography, scintography, and biochemical markers, but they too have limitations in detecting changes in cartilage. MRI concentrates on biochemical composition studies that could yield measures of the sGAG and collagen concentrations, but at this stage the cost is very high and the fixed charge density too low for adequate resolution. As such, clinical MRI approaches do not have the resolution to show early cartilage changes and do not measure physical properties.
Arthroscopy has become an important technique in the diagnosis and therapy of knee OA. A 4 mm diameter arthroscope and/or surgical instruments along with a light source is inserted into the joint capsule, allowing direct visualization of the cartilage surfaces, ligaments, and menisci. The complication rate and morbidity associated with the procedure are so low, that arthroscopy is increasingly being performed on joints that are only minimally symptomatic as an exploratory procedure. It can detect before degenerative changes are evident by radiography. The recent advent of a 1.8 mm diameter needle arthroscope is transforming arthroscopic examination from a hospital-based procedure to a routine office procedure. During a typical diagnostic arthroscopic examination of the knee, the orthopedic surgeon can inspect the AC surface for gross changes. Arthroscopy is performed visually without quantitative biophysical methods. Ultimately, these gross visualization arthroscopic methods, give a visual picture of the cartilage that may or may not correlate with its physical properties.
It is important to note that early OA cartilage may appear normal by visual inspection. Given that arthroscopy is one of the most common orthopedic procedures, visual inspection alone during arthroscopy may not be sufficient for diagnostic purposes, indicating a need for quantitative approaches.
There is currently a commercial arthroscopy blunt probe to subjectively assess the degree of softening (known as xe2x80x9cgrade I chondromalaciaxe2x80x9d) that can result prior to x-ray changes. Another device and method to measure the physical properties of articular cartilage has been developed by Dashefsky (Arthroscopy, 3:80-85, 1987). Dashefsky designed a more objective measurement apparatus. He used an instrumented indenter attached to a force transducer to qualitatively assess the mechanical properties of chondromalacia of patellar cartilage during arthroscopy. In a group of 107 knees with xe2x80x9cpatellofemoral symptoms and signsxe2x80x9d, 90% were evaluated as xe2x80x9csoft;xe2x80x9d but over half of these xe2x80x9csoftxe2x80x9d cartilages showed no detectable visual changes of the articular surface of the patella. Interestingly, of 58 patients with no signs or symptoms of the patella, 50% showed softening of the cartilage. These results suggest that physical property changes may not correlate with the patients"" symptoms until an irreversible threshold of damage occurs with the chronic wear and tear of cartilage.
More recently, Lyyra et al. (Med. Eng. Phys., 17:395-399, 1995), developed an arthroscopic indenter instrument with strain gauges for measurement of tissue stiffness in vivo, produced as Artscan 1000 from Artscan Medical Innovations, Helsinki, Finland. A constant deformation is imposed on the cartilage by the indenter, and the xe2x80x9cinstantaneousxe2x80x9d load response during a one second measurement interval is used to evaluate the tissue stiffness before appreciable stress relaxation has occurred. In order to compute an effective dynamic modulus, an independent measurement of tissue thickness is necessary, as with any indentation technique. The device was able to detect differences in the stiffness of cartilage in different regions of normal knees. Interestingly, however, the indenter detected only 30-40% decreases in cartilage stiffness in the most severely affected regions of the patellar cartilage of patients with known chondromalacia. Use of these devices indicates that purely mechanical tests alone (e.g., indentation tests) may not provide a sufficiently sensitive index of early degenerative changes in cartilage. Because of this, the cartilage""s electromechanical transduction properties have been incorporated into a surface diagnostic probe.
This approach has been termed electromechanical surface spectroscopy and utilizes the current generated stress phenomena of articular cartilage. Spatial and temporal changes in the molecular integrity of the collagen network due to degeneration lead to important changes in the functional mechanical and electrical properties of the tissue and therefore cause changes in the current generated stress. In this approach, illustrated in FIG. 1, small sinusoidal electrical currents are imposed by an interdigitated electrode array 104 that rests on the cartilage articular surface 100. The current causes an electrophoretic motion of the negatively charged cartilage extracellular matrix (ECM) towards the positive electrode and an electroosmotic motion of intratissue fluid towards the negative electrode. These combined effects produce measurable normal mechanical stresses at the tissue surface that can be detected by an overlying piezoelectric stress sensor 102. The stress produced is at the same fundamental frequency as the driving current, but out of phase due to the poroelastic nature of the cartilage response. The penetration depth into the tissue is proportional to the spatial wavelength of interdigitated electrode 104 structure, defined as twice the electrode spacing.
This electromechanical technique is generally described in U.S. Pat. No. 5,246,013. In addition, for this technique, Frank et al. (J. Biomech., 20:629-639, 1987) utilized a uniaxial configuration in which the current was applied via electrodes on opposite ends of a excised cartilage plug. Sachs et al. later completed a mathematical model showing that two silver electrodes placed on the same surface side of cartilage could induce a measurable mechanical response when current is applied in this potentially nondestructive arrangement (Physiochem. Hydrodyn., 11:585-614, 1989 and A Mathematical Model of an Electromechanically Coupled Poroelastic Medium. PhD thesis, Massachusetts Institute of Technology.). In parallel, Salant et al. (Surface Probe for Electrokinetic Detection of Cartilage Degeneration. MD thesis, Harvard-MIT Division of Health Sciences and Technology.) and later Berkenblit et al. (Spatial Localization of Cartilage Degradation Using Electromechanical Surface Spectroscopy With Variable Wavelength and Frequency. PhD thesis, Massachusetts Institute of Technology.) improved on this design by designing a configuration in which current could be applied to a single surface of cartilage and the resultant induced mechanical stress could be measured.
The general technique has also been termed xe2x80x9cimposed-k sensingxe2x80x9d because the medium is excited at a specified temporal (angular) frequency, by an electrode structure having a spatial period=2/k determined by the electrode geometry and hence a dominant wave number k. Its advantages are that it can be made nondestructively (an important requirement for in vivo measurement of cartilage properties) and the electric fields generated decay exponentially into the material, with a penetration depth on the order of /5 to /3. Thus, different depths of the material may be tested by varying the imposed spatial wavelength, and spatial inhomogeneities in material properties can be detected by making surface measurements using a series of imposed spatial wavelengths (spatial localization). The depth to which the current penetrates into the medium is proportional to the effective spatial wavelength, which is equal to twice the center-to-center distance between adjacent electrodes. By changing the imposed spatial wavelength (by having independently addressable electrodes), various depths of the medium can be preferentially assessed.
The use of varying wavelengths is illustrated in FIGS. 2a and 2b, collectively. Both the frequency and wavelength of the applied current density affect the depth of penetration of the current induced poroelastic deformation within the tissue. The characteristic depth of penetration of the current density, itself, is approximately the spatial wavelength of the current. This wavelength, is determined by the electrode excitation pattern at the cartilage surface. Therefore, a probe with four independently addressable electrodes 104 is utilized such that connection to each electrode can be varied externally, thereby enabling multiple wavelengths to be applied using a single device. Applied current densities having short wavelength, illustrated in FIG. 2a, compared to cartilage thickness are confined to the superficial region of the tissue; the associated current generated stress will therefore reflect the properties of the superficial zone. In contrast, long wavelength excitations, illustrated in FIG. 2b, penetrate the full depth of the tissue and thereby reflect the average properties of full thickness cartilage. Thus, combinations of short and long wavelength excitations enable the probe to xe2x80x9cimagexe2x80x9d depth dependent focal lesions.
Spectrometer 102 response depends in a sensitive manner on molecular level changes in the cartilage matrix similar to changes that occur during the earliest phases of OA degeneration. These results provide the fundamental basis for the in vivo surface electromechanical spectroscopic approach to detect cartilage degeneration.
For this method, typically a sinusoidal current density of 1 mA/cm2 is applied to the tissue over the frequency range 0.025-1.0 Hz using a bipolar operational amplifier, such that the total current amplitude is constant at all frequencies, and driven by a programmable frequency generator controlled through a computer. The output of the piezoelectric sensor electrodes are passed through a high impedance electrometer, low pass filtered to remove 60 Hz noise and differentially amplified. The signals are recorded on a computer, and combined with a mechanical sensor calibration done before each test to obtain the current generated stress. By comparison of this current generated stress to the current generated stress in normal cartilage, a determination can be made as to whether the present cartilage has experienced degradation.
While electromechanical spectroscopy is useful in detecting cartilage degeneration, there is still an ongoing need for in vivo techniques to non-destructively and rapidly test for cartilage degeneration. Experiments have shown that use of the electromechanical spectroscopy may be not be able to sensitively measure surface cartilage damage, to a depth of approximately 50 xcexcms, caused by MMP-13. In addition, the most sensitive detection afforded by current generated stress techniques requires measurements at lower frequencies, which require longer measurement time. The present invention overcomes these disadvantages, in addition to providing further advantages when utilized alone, or in conjunction with electromechanical spectroscopy.
A probe for detecting the degeneration of mammalian tissue, particularly articular cartilage, which comprises a pair of electrodes for applying a current to the mammalian tissue is provided. When current is applied to the tissue there is a distribution of current density and an electric field within the tissue, and a resulting voltage difference across the electrodes applying the current. In the preferred embodiment, the voltage difference is measured by a computer, and the computer normalizes the voltage difference to the current applied to the tissue. This normalized parameter is defined as the tissue impedance. The impedance is indicative of the amount of degeneration tissue has undergone.
In an alternative embodiment, electrodes of the probe comprise an interdigitated array of electrodes allowing the wavelength, and hence, the depth of penetration of the current distribution into the tissue to be changed. When a short wavelength mode is utilized, the impedance measurements are representative of a superficial layer""s degeneration. When a long wavelength mode is utilized, the impedance measurements are representative of the bulk tissue degeneration.
In an alternative embodiment, the probe additionally comprises a stress sensor. The current applied to the tissue creates current generated stress in the tissue. This stress is measured and additionally used to determine whether degeneration has occurred. In one embodiment, impedance measurements are taken simultaneously with the current generated stress measurements at the same frequency as the current generated stress measurements. In a different embodiment, the impedance measurements are taken sequentially, i.e. after the current generated stress measurements, at a frequency higher than those at which the current generated stress measurements are made.
A method for detecting the degeneration of mammalian tissue, particularly cartilage, is provided. Current is applied to the tissue creating a distribution of current density and an electric field within the tissue, and a resulting voltage difference across the electrodes applying the current. The amplitude of the voltage difference across the electrodes is measured and divided by the current applied by the electrodes to give the tissue impedance. This tissue impedance is compared to an impedance value of clinically normal tissue to determine if the tissue is degenerated.
In one embodiment, the voltage difference across the electrodes applying current is measured simultaneously to a current generated stress measurement and at the same frequency as the current generated stress measurement. In another embodiment, the voltage difference is measured at a higher frequency than that used for current generated stress measurements, either alone, or after current generated stress measurements have been made.